Regulating environmental temperature is important in decreasing mortality and morbidity for premature infants. Extremely low birth weight (ELBW) infants (infants weighing less than 1000 grams) are vulnerable to cold stress during the first 12 hours of life (transitional period) due to stabilization in the delivery room and neonatal intensive care unit. Previous research has not identified the optimal body temperature range for ELBW infants, which would lead to optimal oxygenation, acid-base balance, stable blood glucose, stable heart rate and that minimizes peripheral vasoconstriction for these infants. This study will use an exploratory, multiple case design to examine the relationship between body temperature and oxygenation, acid-base balance, blood glucose, peripheral vasoconstriction, and heart rate for 10 ELBW infants during the transitional period. Time series analysis will be conducted on physiologic data within subjects to define relationships between temperature and these variables and to look for similarities over infants. Logistic regression within subjects will be used to help define the temperature range predicting normal values of oxygenation, acid-base balance, blood glucose, and heart rate while minimizing peripheral vasoconstriction for ELBW infants.